An update on the dirty dress.....I bought a tube of latex painters caulking in a muddy brown color....squirted it into a tray and mixed in water until it was soupy like, well, mud.  I threw it at the dress which I had hung over a plastic drop cloth and let it run down the fabric.  Then I filled a spray bottle with water and a small amount of caulking (not too much so the sprayer wouldn't clog) and sprayed away at the bigger clumps of mud until they ran and dripped.  When dry...it  looks like a muddy wet dress but the caulking doesn't look like splattered paint and it is flexible enough that it won't crack and flake all over the place.....bet I can even through it in the washing machine and it will still look muddy...

There you go...my recipe for mud